Compatibility layer

From AudioLexic

In software engineering, a compatibility layer allows binaries for an emulated system to run on a host system. This translates system calls for the emulated system into system calls for the host system. With some libraries for the emulated system, this will often be sufficient to run binaries for the host system.

Examples include:

the Linux compatibility layer on the BSDs, which emulates Red Hat Linux using a system call translator and Red Hat libraries

A compatibility layer avoids both the complexity and the speed penalty of full hardware emulation. Some programs may even run faster than the original, e.g. some Linux applications running on FreeBSD's Linux emulation were often held by advocates to perform better than the same applications on Red Hat Linux.

Even on similar systems, the details of implementing a compatibility layer can be quite intricate and troublesome; a good example is the IRIX binary compatibility layer in NetBSD.

A compatibility layer requires the host system's CPU to be (upwardly) compatible to that of the emulated system. Thus, for example, a MS Windows compatibility layer is not possible on PowerPC hardware, since MS Windows requires an x86 CPU; in that case, full emulation is needed.